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19 toiles of the luminous surface of the Sun ; hut, in com- mon with the greater number of Comets, the form of toe part of its orbit traversed in the short time during ' v hich it could be observed, was rather that of a para- llel than an ellipse : and consequently no estimate can oe formed of its aphelion distance. A small number of comets have been found to have e lliptical orbits, and their period of return can be conse- quently predicted; but in general, the orbits seem to be parabolic, so that whether they are really ellipses of very S r eat eccentricity (in which case the return of the comet Bla y he expected), or are really parabolas carrying off the c °inet towards some other system, remains uncertain. Many comets are bodies of extraordinary size and Alliance, especially when they are passing their perihe- The appearance of six of the most remarkable, **^.,1680, 1682,1744,1811,1835 and 1844, is shown in DIAGRAM XIV. Comets generally consist of a large and bright but ul-defined mass of light, called the head, which is Usually much brighter towards the centre, and offers be appearance of a vivid nucleus, like a star or planet, rom the head, and in a direction ppposite to that in ' v hich the Sun is situated, two streams of light appear 0 diverge from the comet. These grow broader and tn °re diffused at a distance from the head, and com- monly close in and unite at a little distance behind but sometimes continue distinct for a great part of beir course, in either case constituting the tail. Some- ttUes the tail is subdivided into several streams of Ominous matter. The tail is generally somewhat j-urved, bending towards the region which the comet as left. It is by no means an invariable appendage 0 comets ; many of the smaller ones appearing only as °und or somewhat oval vaporous masses, increasing in density towards the centre. The planets when seen from the Earth, do not appear